The erbium-doped fiber amplifier (EDFA) is an optical gain medium that is widely used in the telecommunications arena and it possesses an intrinsically low gain per unit length. In optical resonator applications, this forces the need for an extended gain medium, which causes the fundamental cavity frequency to remain rather low (typically 1-10 MHz). Such systems, therefore, require modelocking at extremely high orders to operate at typical (multi-GHz) bit rates.
Harmonically modelocked laser oscillators suffer a unique type of noise first encountered by that is now commonly referred to as supermode noise. See M. F. Becker, D. J. Kuizenga, and A. E. Siegman, xe2x80x9cHarmonic mode locking of the Nd: YAG laserxe2x80x9d, J. Quantum Electron. 8, 687 (1972). This noise results from the statistical independence of individual intracavity pulses, each of which must originate from a slightly different photon noise fluctuation. The resulting small differences between intracavity pulses cause a small amount of amplitude noise to appear at every harmonic of the cavity fundamental.
Suppression of this noise has been attempted lasers using various experimental techniques: incorporation of an intracavity etalon(See G. T. Harvey, L. F. Mollenauer, xe2x80x9cHarmonically mode-locked fiber ring laser with an internal Fabry-Perot stabilizer for soliton transmissionxe2x80x9d, Opt. Lett. 18, 107 (1993)); cavity length dithering at kHz rates(See X. Shan and D. M. Spirit, xe2x80x9cNovel method to suppress noise in harmonically modelocked erbium fibre lasersxe2x80x9d, Electron. Lett. 29, 979 (1993)); and by the use of two-photon absorption to prevent pulse dropouts(See 4. E. R. Thoen, M. E. Grein, E. M. Koontz, E. P. Ippen, H. A. Haus, and L. A. Kolodziejski,xe2x80x9cStabilization of an active harmonically mode-locked fiber laser using two-photon absorptionxe2x80x9d, Opt. Lett. 25, 948 (2000)). However, none of these techniques adequately reduces the noise problems enough.
Various patents have been proposed in the area. See for example, U.S. Pat. No. 5,381,426 to Fontana et al.; U.S. Pat. No. 5,450,427 to Fermann et al.; U.S. Pat. No. 5,590,143 to Takara et al.; U.S. Pat. No. 5,619,320 to Eiselt et al.; U.S. Pat. No. 5,761,228 to Yano; U.S. Pat. No. 5,781,327 to Brock; and U.S. Pat. No. 6,256,328 to Delfyett et al., the latter having the same assignee as that of the subject invention. However, none of the prior art adequately solves the noise problems referenced above.
A primary objective of the present invention is to provide a method and system for ultralow noise and supermode suppression in an actively modelocked external-cavity semiconductor diode laser.
The invention includes a method and system for the suppression of supermode noise in an approximately 10 GHz external-cavity semiconductor diode laser by using a high-finesse intracavity etalon. Using a recently-developed wideband modification of the homodyne noise measurement technique of Derickson, et. al.(D. J. Derickson, A. Mar, and J. E. Bowers, xe2x80x9cResidual and absolute timing jitter in actively modelocked semiconductor lasersxe2x80x9d, Electron. Lett. 26, 2026 (1990).), the inventors have made extended noise sideband measurements out to the Nyquist offset frequency (approximately 5 GHz). Measurements reveal that the supermode suppression is almost total over the entire Nyquist band, attenuating the noise spikes below the approximatelyxe2x80x94140 dBc/Hz white noise floor of the system. The invention also compares the noise of this harmonically-modelocked laser with a similar fundamentally-modelocked laser (also operating at approximately 10 GHz) to illustrate the advantages and disadvantages of each approach.